Single stage cleaning of metal surfaces with aqueous cleaning solutions is known. The cleaning solutions remove coolants and lubricants employed in machining operations. Metal fines and other contaminants from metal forming and machining operations are also removed by the cleaning process. Conventional cleaners frequently result in a surface finish which is susceptible to oxidation. For metal articles which are machined to close tolerances, such surface degradation can have a deleterious effect on later operations, such as automated assembly operations.
Typically, aluminum parts undergo a series of machining, polishing, stamping, etc., steps prior to being considered a finished part. After each step, the processing soils must be removed so that the parts can be tested and inspected prior to subsequent operations. The cleaners which are used to remove these soils must not stain, etch or discolor the metal. In addition, other requirements may exist, which are specific to a particular operation.
For aluminum cleaning, it is important that the cleaner not stain or darken the surface. Aside from the darkening being aesthetically unpleasing, it is an indication of the surface being attacked or dissolved and oxide formation occurring. Many aluminum parts being cleaned are highly polished, and any appearance of an oxide or darkening is a source for rejection. Many cleaners in current use for cleaning aluminum contain alkanolamines or phosphates. These substances tend to leave a stain or haze on the aluminum part, or etch the surface.
The requirements of a single stage cleaning process are substantially different than other metal cleaning operations. Only a single pH may be used. Intermediate rinses are not available to remove soils and metallic fines. The cleaning solution must drain efficiently from the cleaned surface. Also, the growth of oxide layers on the metal surface must be controlled after oils have been removed by the cleaner. The current use of chlorinated solvents to accomplish these goals is under increasing pressure from both health and environmental regulations.